Spaceflight Insider

Pluto has blue hazes and surface water ice

Blue skies on Pluto

Pluto’s Blue Sky: Pluto’s haze layer shows its blue color in this picture taken by the New Horizons Ralph/Multispectral Visible Imaging Camera (MVIC). The high-altitude haze is thought to be similar in nature to that seen at Saturn’s moon Titan. The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles (called tholins) that grow as they settle toward the surface. This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible. Image & Caption Credit: NASA/JHUAPL/SwRI

NASA’s New Horizons spacecraft has revealed the first color images of Pluto’s atmospheric hazes showing that its sunrises and sunsets are colored blue, according to recent data received last week. The spacecraft has also revealed small regions of exposed water ice on Pluto’s surface.

Images taken by the spacecraft’s Ralph/Multispectral Visible Imaging Camera (MVIC) show a ring of blue hazes surrounding Pluto against the black background of space.

Mission scientists combined data from blue, red, and near-infrared images taken at the spacecraft’s July 14 closest approach, processing it via software to replicate as closely as possible the view an observer on Pluto’s surface would see.

Sunrises and sunsets would appear blue to a person standing on Pluto. Scientists are uncertain as to whether a blue sky would be visible during the day because the hazes are so thin, said mission scientist Carly Howett.

Using the latest images, Howett created a video showing variations in the dwarf planet’s layers of haze:


Pluto’s high altitude haze is believed to be similar to that seen on Saturn’s moon Titan. On both worlds, the hazes are produced by chemical reactions of nitrogen and methane initiated by sunlight. The way the haze particles scatter blue light helps scientists determine the haze’s composition and size.

“A blue sky often results from scattering of sunlight by very small particles,” Howett explained. “On Earth, these particles are very tiny nitrogen molecules. On Pluto, they appear to be larger – but still relatively small – soot-like particles we call tholins.”

Tholins, which usually have a reddish color, are produced when ultraviolet sunlight breaks methane and nitrogen molecules apart in the upper atmosphere. These ionized particles then interact with one another to form more complex ions, some positively charged and others negatively charged.

The larger, more complex molecules then recombine and grow to form macromolecules and eventually small particles. During this process, volatile gases condense and coat the particles with ice frost. The particles fall back through the atmosphere onto the planet’s surface, giving that surface its reddish color. This process was first discovered in Titan’s upper atmosphere.

Principal investigator Alan Stern said scientists are studying the newest data to determine whether the blue sky can be seen at all times of day from Pluto’s surface.

“Who would have expected a blue sky in the Kuiper Belt? It’s gorgeous,” Stern said.

The discovery of surface regions with exposed water ice constitutes a second significant finding from last week’s data. Mission team members combined surface images taken by the Ralph instrument in visible light using MVIC and via infrared spectroscopy using the Linear Etalon Imaging Spectral Array (LEISA), to produce a composite image that highlights regions of exposed water ice in blue.

Water Ice on Pluto

Water Ice on Pluto: Regions with exposed water ice are highlighted in blue in this composite image from New Horizons’ Ralph instrument, combining visible imagery from the Multispectral Visible Imaging Camera (MVIC) with infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA). The strongest signatures of water ice occur along Virgil Fossa, just west of Elliot crater on the left side of the inset image, and also in Viking Terra near the top of the frame. A major outcrop also occurs in Baré Montes towards the right of the image, along with numerous much smaller outcrops, mostly associated with impact craters and valleys between mountains. The scene is approximately 280 miles (450 kilometers) across. Note that all surface feature names are informal. (Click to enlarge.) Image & Caption Credit: NASA/JHUAPL/SwRI

The image above depicts an area approximately 280 miles (450 kilometers) across. Areas that show the strongest signatures of water ice include Virgil Fossa, just west of Elliot crater on the left side of the inset image, Viking Terra close to the top of the picture, and Baré Montes toward the right.

Evidence of water ice is also present in many smaller regions, in valleys between mountains, in smaller outcrops, and in areas associated with impact craters. In various locations, water ice is hidden by other ices.

“Large expanses of Pluto don’t show exposed water ice because it’s apparently masked by other, more volatile ices across most of the planet. Understanding why water appears exactly where it does, and not in other places, is a challenge that we are digging into,” science team member Jason Cook of the Southwest Research Institute (SwRI) said.

Scientists are also baffled by the fact that the patches of water ice appear bright red. Science team member Silvia Protopapa of the University of Maryland at College Park acknowledged this development is surprising.

“We don’t yet understand the relationship between water ice and the reddish tholin colorants on Pluto’s surface,” Protopapa said.

The images below of Pluto’s small moons Nix and Hydra show Nix to be a white color with a large red crater on its surface. Hydra has a complicated shape, resembling a larger version of Comet 67P/Churyumov-Gerasimenko.

Pluto's moons Nix and Hydra

On the left, Pluto’s moon Nix is shown in high-resolution black-and-white and lower resolution color. On the right, Pluto’s moon Hydra. Image Credit: NASA/JHUAPL/SwRI



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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